This invention relates to loudspeakers, and, more particularly, to a method and apparatus for varying the motor strength of a loudspeaker to alter its frequency response and damping characteristics.
Loudspeakers generally comprise a frame, a motor structure, a diaphragm connected to an upper suspension or surround and a spider or lower suspension. In one common type of speaker, the motor structure includes a permanent magnet mounted between a top plate and a back plate. A pole piece is centrally mounted on the back place and concentrically disposed within a bore formed in the permanent magnet and an aligning bore formed in the top plate. A space is provided between the top plate and pole piece defining a magnetic gap within which lines of magnetic flux are produced by the permanent magnet. One alternative speaker design, commonly known as a xe2x80x9cpotxe2x80x9d type speaker, employs a somewhat different motor structure in which the pole piece is centrally disposed on and connected to the back plate, while a permanent magnet is connected between the pole piece and top plate. A pot wall concentrically surrounds the pole piece, magnet and top plate, and forms a magnetic gap with the top plate within which lines of magnetic flux are produced by the permanent magnet. In both types of speakers, a voice coil is provided including a hollow, cylindrical-shaped former having an outer surface which mounts a winding of wire.
In each speaker design noted above, one end of the diaphragm is connected to the surround, which, in turn, is mounted to the upper end of the frame. The spider is connected at one end to a seat formed in the frame at a point between its upper and lower ends. The free ends of the diaphragm and spider are mounted to the voice coil and support it within the magnetic gap in the motor structure such that the former of the voice coil is concentrically disposed about the pole piece and the voice coil winding is axially moveable within the magnetic gap.
The permanent magnet associated with the motor structure or driver in each of the speakers described above produces a xe2x80x9cstaticxe2x80x9d or essentially constant DC magnetic flux within the magnetic gap. In the course of operating a loudspeaker, electrical current is supplied to the wire winding of the voice coil, which, in turn, is located in the magnetic gap. When energized, the voice coil produces an AC flux which alternately has a positive value and a negative value, both of which are less than the static DC flux produced by the permanent magnet.
The AC flux from the voice coil acts with the DC flux within the magnet to cause axial movement of the voice coil within the magnetic gap.
For one polarity of current supplied to the voice coil the magnetic flux from the magnet is reinforced, while energizing the voice coil with current of the opposite polarity causes the voice coil to develop a magnetic flux which opposes that of the magnet. Hence, the voice coil flux modulates the static flux produced by the magnet within the magnetic gap.
The degree of force applied by the motor structure of a loudspeaker, which is an indication of the strength of the motor, is defined by the following relationship:
If the current I through the voice coil is one (1) ampere or normalized to one (1) ampere, then the product of B and L represents the force per ampere applied by the motor structure of the loudspeaker on the voice coil. The term xe2x80x9cLxe2x80x9d specifically refers to the total length of the wire forming the wire winding carried on the outer surface of the former of the voice coil. As a practical matter, the magnetic flux produced by the voice coil has a greater effect when the polarity is such that it reinforces the DC magnetic flux of the magnet than when its polarity is reversed. As a result, the force exerted by the motor structure on the voice coil, or BL (per ampere) is different depending on the polarity of the current to the voice coil. This modulation is a clear source of distortion in the performance of most loudspeakers.
In addition to problems with distortion created by modulation of the static magnetic flux within the magnetic gap, motor structures in conventional loudspeakers have no means for altering their frequency response or damping characteristics. As a result, the acoustic output of such speakers is fixed unless an attempt is made to alter the loudspeaker enclosure. This is often impractical, and in many instances the speaker may not be capable of providing the desired response even in a new enclosure due to fundamental speaker characteristics. In order to meet varying needs, a wider variety of speakers may be required which is too costly for many consumers.
It is therefore among the objectives of this invention to provide a motor structure for a loudspeaker which is adjustable in strength to permit variation of the motor""s frequency response and damping characteristics, which is capable of operating with different types of loudspeakers including pot-type speakers, which is simple in construction and economical to implement.
These objectives are accomplished in a loudspeaker having a motor structure which incorporates a magnetic flux control system including a field winding, a voltage or resistance controller connected between a voltage source and the field winding and, a polarity reversal switch preferably connected across the field winding. The magnetic flux control circuit is operative to produce a magnetic flux, which, depending on the level and polarity of electrical current supplied to the field winding, either reinforces or opposes the static magnetic flux produced by the magnet of the motor structure of the loudspeaker.
This invention is predicated on the concept of controlling the acoustic output of a loudspeaker by providing a motor structure whose strength can be varied on the order of about plus or minus twenty percent to accommodate a comparatively broad range of operating conditions. If the current supplied to the field winding results in a magnetic flux which reinforces the magnetic flux of the permanent magnet of the motor, the motor strength increases thus increasing the damping effect of the motor. Conversely, developing a magnetic flux in the field winding which opposes the static magnetic flux of the permanent magnet decreases the motor strength and reduces the damping effect of the motor.
In one presently preferred embodiment, the motor structure conventionally includes a back plate, a pole piece centrally mounted on the back plate, and, a top plate and permanent magnet concentrically disposed about the pole piece. A magnetic gap is formed between the top plate and pole piece across which lines of magnetic flux are produced by the permanent magnet. In this embodiment, the field winding is either mounted directly to the pole piece, or the field winding is a free-standing structure mounted to the back plate in the space between the pole piece and magnet. The field winding is positioned to generate lines of flux which are effective to reinforce or oppose those produced by the permanent magnet within the magnetic gap.
In an alternative embodiment, a xe2x80x9cpotxe2x80x9d type motor structure employed including a back plate, a pole piece centrally mounted on the back plate, a permanent magnet mounted atop the pole piece and a top plate mounted to the magnet. A pot wall is circumferentially disposed about the pole piece, magnet and top plate forming a magnetic gap with the top plate. In this embodiment, the field coil is either mounted to the pole piece or to the pot wall and produces lines of magnetic flux which either oppose or reinforce those created by the permanent magnet within the magnetic gap.
Regardless of the type of motor structure employed in the speakers of this invention, the magnetic flux control system determines the level and polarity of the current supplied to the field winding. In one presently preferred embodiment, the magnetic flux control system comprises a polarity reversal switch and a controller in the form of an adjustable voltage regulator located between a source of voltage and the field winding. The adjustable voltage regulator effectively regulates the level of voltage supplied to the field winding, whose polarity is changed by the polarity reversal switch, thus providing a comparatively large variation in the magnetic flux produced by the field winding. In an alternative embodiment, the controller of the magnetic flux system comprises a number of lines each containing a different resistor, or no resistor at all, connected to a multi-position switch. The lines are arranged in parallel to one another but are serially connected between a voltage source and the polarity reversal switch, which, in turn, connects to the field winding. The multi-position switch is effective to form a completed circuit between the voltage source and field winding, via any one of the lines, thus introducing a different line resistance, which, in turn, alters the current level supplied to the field winding.